1. Field of the Invention
This invention relates to an electrophotographic printer and, more particularly, to an electrophotographic printer adapted to print a toner image adhered electrostatically to a photosensitive member such as a photosensitive drum on the lower surface of a sheet by transferring the image thereto.
2. Description of the Prior Art
FIG. 2 shows an example of an electrophotographic printer. Sheet cassettes 3 for containing sheets 2 of different sizes are detachably attached to one end of a printer body 1 of box shape as a housing, and a hopping roller 4a for taking one by one the sheets 2 in the cassettes 3 and a feed roller 4b for conveying the sheets 2 are arranged near the cassettes 3. A sheet conveyor for connecting the cassettes 3 to an exhausted sheet tray 29 to be described later is disposed on a substantially horizontal flat surface. A cylindrical photosensitive drum 5 is disposed in the body 1. On the outer periphery of the photosensitive drum 5 are sequentially arranged according to the rotating direction of the drum a corona charger 6 as a state control mechanism for uniformly applying charge to the photosensitive drum 5 to set the drum 5 to a writable state, an LED head 7 as an optically writing mechanism for forming an electrostatic latent image on the photosensitive drum 5, a developing unit 8 for depositing toner to the electrostatic latent image formed on the photosensitive drum 5 to visualize the latent image, a transfer unit 9 for transferring the toner to the lower surface of the sheet 2, a separating corona charger 10 disposed adjacently to the downstream side of the sheet 2 of the transfer unit 9 with respect to the conveying direction for separating the sheet transferred with the toner from the photosensitive drum 5, an electric eliminator 11 for removing the remaining potential on the photosensitive drum 5, and a cleaner 15 formed of a brush 12 for removing the remaining toner on the photosensitive drum 5 and a collector roll 14 for recovering the toner dropped by the brush 12 and storing the toner to a lower section disposed with a collector spiral 13.
A registration roller 16 for conveying the sheet 2 fed from the feed roller 4b between the photosensitive drum 5 and the transfer unit 9 is arranged at the upstream side of the drum 5 with respect to the conveying direction of the sheet 2, and a suction conveying mechanism 18 for conveying the sheet separated from the photosensitive drum 5 by the separating corona charger 10 to a fixing unit 17 is arranged at the downstream side of the drum 5 with respect to the conveying direction of the sheet 2. The suction conveying mechanism 18 is formed, as shown in FIGS. 3 and 4, of a suction mechanism 19 and a conveying mechanism 20. One suction mechanism 19 is provided on its lower surface with a suction fan 22 in a box 21 of the size from the separating corona charger 10 to the fixing unit 17, and a number of suction holes 23 are opened at the lower surface of the box 21. The other conveying mechanism 20 is formed by exposing downward endless conveying belts 25 engaged between a pair of rotary rollers 25 and 25 from a plurality of long holes 24, 24 opened in parallel with the direction perpendicular to the conveying direction of the sheet 2 on the lower surface of the box 21. The fixing unit 17 contains a heat roll 27 and a rubber roller 28 rolling by pressing by the roll 27, and an exhaust sheet tray 29 for receiving the fixed sheets 2 is attached to the outlet side of the body 1.
In the electrophotographic printer described above, the sheet 2 of either one size of the cassettes 3 are selected, the sheet 2 is taken by the hopping roller 4a, and further conveyed toward the registration roller 16.
Then, after the photosensitive drum 5 is set by the corona charger 6 to a writable state, an electrostatic latent image is formed on the photosensitive drum 6 by the LED head 7 according to a predetermined print signal. Then, toner is adhered to the electrostatic latent image formed on the photosensitive drum 5 by the developing unit 8, and the toner is transferred to the lower surface of the sheet 2 fed by the registration roller 15 by the transfer unit 9. Thereafter, the sheet 2 is separated from the photosensitive drum 5 by the separating corona charger 10. Subsequently, the suction conveying mechanism 18 is driven to feed the sheet toward between the heat roller 27 and the rubber roll 28 of the fixing unit 21 while sucking the upper surface of the sheet to the lower surface of the conveying belt 25 of the conveying mechanism through the suction hole 23 by the negative pressure of the suction fan 22 of the suction mechanism 19. The sheet 2 is passed through the fixing unit 17 to be fixed with the toner image transferred thereby, and exhausted on the external exhausted sheet tray 29 of the printer body 1.
On the other hand, the photosensitive drum which has thus transferred is eliminated at the remaining potential on the surface thereof by the electric eliminator 11, the remaining toner on the surface of the photosensitive drum 5 is dropped by the brush 12, and the photosensitive drum 5 is then set again to a chargeable state. The toner dropped by the brush 12 is stored in the lower section by the cleaner 15 disposed with the collector spiral 13 by the collector roll 14.
However, in the printer of the constructiton described above, the transferred sheet 2 is sucked by the negative pressure from the suction hole 23 to the lower surface of the conveying belt 26 of the suction conveying mechanism 18 to be conveyed. Therefore, when an image is transferred to a normal sheet, the sheet can be preferably conveyed, but when the image is transferred to an envelope, it has such a problem that the envelope cannot be stably conveyed. More specifically, as shown in FIGS. 5(a) and 5(b), a cover 31 is foldably formed at one end edge of an envelope body 30 and the cover 31 is held in a state floated from the envelope body 30. Thus, as shown in FIG. 6, when it is sucked to the conveyor belt 26, only the cover 31 of th envelope body 30 is sucked, but the envelope body 30 cannot be suitably sucked. Therefore, it has a problem that the envelope body 30 cannot be positively sucked to the lower surface of the conveying belt 26 to be conveyed. Particularly, when the envelope body 30 is conveyed with the cover 31 disposed forward in a landscaping direction, this problem frequently occurs.